The present invention relates to a cutting insert having a polygonal basic shape, for chip forming machining, preferably milling, comprising two mainly parallel faces and edge surfaces connecting them. Lines of intersection between two edge surfaces and at least one face form main cutting edges. A plurality of chamfers are formed at the transition between said edge surfaces in a corner portion of the insert. A first chamfer forms a minor cutting edge at its line of intersection with said face.
It is previously known at inserts for milling to provide the corner portions of the insert with a corner chamfer between associated minor and main cutting edges, the external angle between said minor cutting edge and said main cutting edge being approximately half as large as the external angle between the minor cutting edge and the main cutting edge. This geometry gives a relatively sharp corner between the corner chamfer and the main cutting edge. During milling operations the chip is forced perpendicularly to each active cutting edge and therefore the part of the chip generated by the main cutting edge collides with the part of the chip generated by the corner chamfer. The sharp corner between the corner chamfer and the main cutting edge is thus exposed to an increased load relative to the corner chamfer and the main cutting edge and thus becomes the part of the insert which firstly is cracked or deformed. It is also known to provide the transition between the minor cutting edge and the main cutting edge with a rounded-off portion, having a constant radius. Since the generated chip is guided perpendicularly to the edge of the portion and the main cutting edge also in this case a compression of the chip arises at the transition between the rounded-off portion and the main cutting edge.
The present invention relates to a cutting insert through which shape the above-mentioned drawback is avoided. A favourable upset of the chip is achieved at the cutting edges of the corner portion and reduced forces are achieved at the corner between the corner chamfer and the main cutting edge during the machining.